1. Field of the Invention
This invention relates to a detector and a method to accurately and quickly detect the oxygen concentration in a gaseous mixture at a comparatively low temperature with a high-speed response and a low output impedance.
2. Description of the Prior Art
To measure the oxygen concentration of the exhaust gas from an internal combustion engine or the like, an oxygen concentration detector having an oxygen ion conductive solid electrolyte body and based on the principle of an oxygen concentration cell has been used. The oxygen concentration detector of the prior art generally includes a solid electrolyte body, which is for instance made of a yttria zirconia ceramic, and electrodes made of platinum. The aforementioned oxygen concentration detector of the prior art has shortcomings in that, when the temperature of the gaseous mixture being measured is comparatively low, the catalytic activity of the platinum electrode is reduced and the electric resistance of the zirconia ceramic becomes high, resulting in an increased impedance of the oxygen concentration detector which tends to make output signals of the oxygen concentration detector suitable due to interference by noises, and that the comparatively low temperature of the gaseous mixture tends to make the response slow. As a result, the practicable lowest operative temperature of the oxygen concentration detector of the prior art has been about 350.degree. C. On the other hand, at the starting of an internal combustion engine and during idling thereof, the temperature of the exhaust gas therefrom can be as low as 250.degree. to 300.degree. C., which is below the aforementioned lowest operative temperature of the prior art. Accordingly, the conventional oxygen concentration detector has a drawback of not providing satisfactory operation when the temperature of the gaseous mixture being measured is low.